1. Field of the Invention
The present invention relates to a thin-film forming apparatus which is employed for the fabrication of a display device having an EL (electroluminescence) element (hereinbelow, termed xe2x80x9cEL display devicexe2x80x9d), and to a method of fabricating the EL display device as employs the thin-film forming apparatus.
2. Description of the Related Art
In recent years, researches have been vigorously made in EL display devices each of which has an EL element as a spontaneous emission type element. In particular, notice has been taken of an organic EL display device which employs an organic material as an EL material. The organic EL display device is also called the xe2x80x9corganic EL display (OELD)xe2x80x9d or xe2x80x9corganic light emitting diode (OLED)xe2x80x9d.
Unlike a liquid crystal display device, the EL display device is of spontaneous emission type and therefore has the merit of involving no problem on a view angle. In other words, the EL display device is more suitable than the liquid crystal display device as a display which is used outdoors, and its application in various forms has been proposed.
The EL element has a construction in which an EL layer is sandwiched in between a pair of electrodes, and in which this EL layer has a multilayer structure ordinarily. Typically mentioned is the multilayer structure of xe2x80x9chole transporting layer/light emitting layer/electron transporting layerxe2x80x9d proposed by Tang et al., Eastman Kodak Company. The multilayer structure exhibits a very high emission efficiency, and most of the EL display devices being currently under research and development adopt this structure.
Herein, a predetermined voltage is applied to the EL layer of the above structure by the pair of electrodes, whereby light is emitted owing to the recombination of carriers taking place in the light emitting layer. There are two sorts of schemes for the light emission; a scheme wherein the EL layer is formed between two stripe-like electrodes disposed orthogonally to each other (simple matrix scheme), and a scheme wherein the EL layer is formed between pixel electrodes connected to TFTs and arrayed in the shape of a matrix, and a counter electrode (active matrix scheme).
Meanwhile, the EL materials of the hole transporting layer, the light emitting layer, etc. are broadly classified into two; a low molecular material, and a high molecular material. While materials mainly including Alq3 have been known for a low-molecular light emitting layer for a long time, a high-molecular (polymeric) light emitting layer has been noticed especially in Europe in recent years. Typically mentioned are PPV (polyphenylene vinylene), PVK (polyvinyl carbazole), polycarbonate, etc.
The reasons why the high-molecular EL material is noticed are the points that it can be formed into the layer by a simple method of forming a thin film, such as spin coating process (also termed xe2x80x9csolution application processxe2x80x9d), dipping process, printing process or ink jet process, and that it is higher in the thermal stability as compared with the low molecular material.
Usually, the low-molecular EL material is formed into the layer by vacuum evaporation. That is, ordinarily the EL material is successively stacked without breaking a vacuum in a vacuum evaporator. Besides, an electrode of small work function is employed as the electrode acting as the cathode of the EL element, and also this cathode is ordinarily formed in succession to the EL material.
The EL material is extraordinarily liable to oxidize, and the oxidation is readily promoted even by the presence of a slight water content till the degradation of this EL material. In case of forming the EL element, therefore, the surface of the anode thereof being the lowermost layer is first preprocessed to eliminate moisture etc., whereupon the EL material and the cathode are successively formed on the anode without breaking a vacuum. On this occasion, the EL material and the cathode are sometimes deposited on the selected parts of the anode by employing a shadow mask or the like, and all the processing steps are performed in an evacuated processing chamber even in such a case.
This holds true also of the high-molecular EL material. Even in case of the spin coating process etc. which are not a thin-film forming expedient in a vacuum, it is important for the suppression of the degradation of the EL material that the EL material is prevented from being exposed in the atmospheric air which contains moisture.
The present invention has been made in order to fulfill the above requirements, and has for its object to provide a thin-film forming apparatus which is the most favorable for the fabrication of an EL display device employing a high-molecular EL material.
Another object is to provide a method of fabricating an EL display device of high reliability by utilizing such a thin-film forming apparatus.
The purport of the present invention consists in that an EL display device is fabricated using a thin-film forming apparatus of multi-chamber system (also termed xe2x80x9ccluster tool systemxe2x80x9d) or in-line system which integrally comprehends means for forming a thin film made of a high-molecular EL material (hereinbelow, the thin film shall be sometimes termed the xe2x80x9chigh-molecular EL layerxe2x80x9d), and means for forming a cathode.
While there are various methods of forming the film of the high-molecular EL material, the adoption of a spin coating process is favorable. The spin coating process is an expedient wherein a solute to become the main component of the thin film is dissolved in a solvent, the resulting solution is applied by a spinner or the like, and the solvent is subsequently volatilized by a baking process, thereby to form the thin film.
In the present invention, the solution containing the high-molecular EL material is applied by the spinner, and the solvent is volatilized by performing a heat treatment at a temperature at which the high-molecular EL material is not crystallized (concretely, at a temperature which is not higher than a glass transition temperature). As a result, the high-molecular EL layer is formed on a substrate. In other words, the formation of the high-molecular EL layer necessitates means for applying the solution containing the high-molecular EL material, and means for baking after the application.
Besides, the high-molecular EL material is weak against oxygen likewise to a low-molecular EL material, and an electrically conductive film to be provided after the formation of the high-molecular EL layer should desirably be formed so that this high-molecular EL layer may not be exposed to an environment containing moisture or oxygen. It can accordingly be said that the means for forming the high-molecular EL layer (in the present invention, means for executing the spin coating process), and the means for forming the conductive film to serve as the cathode (or an anode), on the high-molecular EL layer (means for executing a vapor-phase film formation process such as vacuum evaporation or sputtering) should desirably be installed in the identical thin-film forming apparatus.
The present invention achieves the above requirements by the thin-film forming apparatus of multi-chamber system, and consists in technology for fabricating the EL display device of high reliability by adopting such a thin-film forming apparatus.